The present invention relates to wound field synchronous machines (e.g. electrical motors and generators) and in particular to a wound field synchronous machine using capacitive coupling to transfer electrical power to the rotor.
Electrical motors and generators share similar structures of a magnetically interacting stator and rotor and may be collectively termed “electrical rotating machines.” Electrical rotating machines employing permanent magnets for the rotor are termed permanent magnet synchronous machines (PMSM) and are popular in high-volume traction applications (e.g. motor drives for hybrid vehicles) and for compact electrical generation (e.g. generators used in wind turbines) because of their high torque density and efficiency.
The permanent magnets in PMSMs typically use rare earth materials of limited supply whose extraction and refinement may inflict detrimental effects on the environment. For this reason, wound field synchronous machines (WFSM), using an electrical coil in place of the permanent magnet on the rotor, have received renewed attention. WFSMs have lower torque density in comparison to PMSMs but, by permitting control of the rotor field directly, allow more sophisticated motor control, for example, permitting high power factor throughout the machine operating range of different operating speeds and torques. The ability to control the rotor field also permits improved handling of faults by allowing back EMF to be controlled (by removing the field current).
A significant disadvantage to WFSMs is difficulty of coupling significant electrical power to a rotating rotor coil. Such coupling may be done by using electrical “slip rings” in which brushes, typically a carbon composite material, mechanically slide on continuous or semi-continuous metal rings. This mechanical approach is subject to problems of wear on the brushes and rings and the problem of generating debris from such, which may contaminate the environment of the motor.
An alternative approach to mechanical electrical coupling is the transfer of electrical energy by mutual inductance between coils of a rotary transformer. In such a transformer, a stationary primary coil may communicate, via magnetic fields, with a secondary coil mounted to rotate with the rotor. The conductive coils of a rotary transformer and the ferromagnetic components normally used to concentrate the magnetic flux may substantially increase the weight and cost of the motor.
Capacitive coupling is known for low-power electrical data transfer, for example, for transferring digital data from a rotating device. The use of capacitive coupling in motor applications for example, by using adjacent rotating and stationary capacitor plates, is hampered by relatively small capacitance that can be obtained with practical tolerances and the need for significantly greater amounts of power for practical motor operation.